A major aim of this grant is to investigate the developmental origin of the skeleton-forming cells in the head, as well as their ability to regenerate craniofacial skeleton in adults after injury. The head skeleton derives from a special population of cells, the neural crest, which has the remarkable ability to form not only neurons but also skeletal tissues. In the previous grant cycle, we published a manuscript in PLoS Genetics describing the role of a variant histone H3.3 protein in controlling the ability of neural crest cells to form the head skeleton of zebrafish. As histone H3.3 is a core component of the chromatin around which DNA is wrapped, our findings suggest a novel mechanism by which changes in chromatin structure endow the neural crest with the ability to form a wide array of derivatives. In addition, we published a separate study in PLoS Genetics that showed a critical role of Twist1 in guiding these neural crest cells to make head skeleton at the expense of other cell types such as neurons. Together, our studies of H3.3 and Twist1 in zebrafish will shed light on how to generate cells with the ability to form replacement head skeleton in patients. In ongoing experiments, we are using principles from our zebrafish system to directly convert mammalian cells (initially in mouse but then in humans) to a neural crest and skeletal fate.
A parallel strategy that we are taking towards regenerative strategies for facial skeleton is to stimulate endogenous neural crest cells to make replacement skeleton. We have a limited ability to repair defects in our skeleton, for example after bone fracture. However, we have found that adult zebrafish have the remarkable ability to regenerate nearly their entire lower jaw following amputation. By studying why zebrafish regenerate facial skeleton to a much greater extent than humans, we hope to devise molecular strategies to augment skeletal repair/regeneration in patients. In particular, we have found that during zebrafish lower jawbone regeneration, an unusual cartilage intermediate is able to directly make replacement bone, which is in marked contrast to the way bone is made during development. Furthermore, we have found a potentially critical role of the Ihh signaling pathway in allowing regenerating cartilage cells to directly make replacement bone. In the coming period, we plan to test the functional requirements of Ihh signaling in mediating jaw regeneration, as well as identifying the cellular source of bone-producing cartilage cells during jaw regeneration. As similar bone-producing cartilage cells may also be present in human fractures, lessons learned from zebrafish may allow us to stimulate these cells and hence augment bone repair in patients.